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Twenty-six police officers have gone on trial in Brazil for the killing of 15 inmates in one of the country's most infamous prisons two decades ago.

The 1992 military operation in São Paulo's Carandiru prison left more than 100 prisoners dead. The authorities claimed the police were trying to break up a fight between prisoners who had seized control of the jail's Block 9.

But evidence uncovered later suggested that the military police had shot prisoners and that after the massacre they destroyed evidence which could have determined individual responsibility for the killings.

No one is currently in prison for the murders. The only individual to face a trial, Colonel Ubiratan Guimarães, was acquitted in 2006.

Carandiru - which at the time was one of the largest prisons in Latin America - was shut down in 2002 after inmates in various Brazilian prisons held a series of coordinated riots to protest at conditions in Brazilian jails.

'Shoot first, ask later'

The Carandiru massacre didn't come as a surprise to many human rights organizations, including Amnesty International. Even the local authorities say Brazil's prison system has long been "at breaking point".

"The Carandiru massacre is a reflection of the deep problems of Brazil's prison system," said Atila Roque, Director of Amnesty International's Brazil Office.

"For many decades, we have witnessed a toxic recipe of inhuman detention conditions mixed with the 'shoot first, ask later' policy that seems to have been adopted by police in São Paulo."

Amnesty International has, over many decades, documented thousands of cases of violence by São Paulo's military police - including cases of executions in prisons across the state.

The massacre at Carandiru follows a history of impunity for the extrajudicial executions of inmates in previous prison riots and of civilians on the streets of São Paulo.

Prisons at breaking point

Experts say the shockingly poor state of Brazil's prisons is a big part of the problem.

According to country's Penitentiary Department at the Ministry of Justice, Brazil's prison population has more than doubled over the past 10 years - from 233,000 in 2000 to 513,802 in June 2011 - and continues to rise.

Despite massive investment by the federal government of around half a billion dollars between 2003 and 2009, prison building has not kept pace with the increasing number of detainees.

By the end of 2012, there was a shortfall of 200,000 places in prisons across the country, which led to severe overcrowding and inhumane living conditions.

'Medieval prison system'

The state of Brazil's prisons is so dire that even the country's Minister of Justice, José Eduardo Cardozo, remarked in November of 2012: "We have a medieval prison system, which not only violates human rights, it does not allow for the most important element of a penal sanction, which is social reintegration."

More recently, during a visit to the Amazonas state in north-western Brazil, Amnesty International's delegates witnessed a catalogue of prison abuses and saw inmates in foetid, overcrowded, insecure cells.

In several prisons women and minors were detained in the same units as men, and there were numerous reports of torture, including near-suffocation with a plastic bag, beatings and electric shocks by the state military police.

On the day delegates visited the Cadeia Pública Raimundo Vidal Pessoa - which the authorities have recommended for closure on numerous occasions - 900 detainees were held in the men's wing which was designed to hold 104 people and there were 208 women detained in cells designed to accommodate 35.

In some prisons in Amazonas state, toilets are simply holes in the ground shared by up to 10 or 15 cell mates. In some facilities, delegates saw open sewers and rotting food and rubbish in corridors and yards. In Tefé, prisoners complained that the septic tank flooded and filled their cell with noxious waste.

These conditions have contributed to health problems among prison staff. Both prison guards and detainees complained of ringworm ("micose") which the overcrowded and insanitary conditions helped spread through the system.

Chain of command responsibility

Within days of the Carandiru massacre, an Amnesty International team visited the prison to gather evidence that was used to compile a unique record of what had occurred in each of the cells in Block 9.

They were also able to evaluate the forensic evidence and document the grossly inadequate handling of ballistic evidence, which amounted to an attempted cover-up by officials.

The report Brazil: "Death has arrived" included strong evidence that São Paulo's Governor and Secretary of Public Security abdicated their responsibilities by handing complete control of the prison to the military police and were therefore also responsible for what happened.

"There's nothing preventing the Carandiru massacre from happening again unless authorities at the state and federal level take responsibility for allowing poor police practices to go unchecked and letting people rot in prisons," said Atila Roque.